1. Field of the Invention
This invention relates to a process for the tightening of a joint having a threaded joint element, for applying a predetermined tightening force to this joint.
2. Description of the Prior Art
All the tightening processes currently used for holding joints together by means of a threaded joint element operate by applying to these joints a predetermined tightening force on which the holding of the joint depends during its use.
Among the known tightening processes, there are:
Torque screwing. This is by far the most used method. It consists of screwing the joint element until a given resisting torque is obtained. It is easy to use since all that is required is a torque wrench or, for automation on the assembly line, a torquemeter is placed in the screwing assembly line and screwing is stopped when the specified torque has been measured. Unfortunately, variations in the prestressed force are very large because the friction coefficient upon which the torque resistance depends is extremely variable. It has been attempted to eliminate this serious drawback by using a solid lubricant at the site of the threads, such lubrication resulting from the application of special varnishes, but the treatment of the bolt and the nut greatly increases the cost of the joint.
Angle screwing. The principle of angle screwing is simple. It consists of detecting the contact of the parts of the joint by observing the rise in torque, then in screwing by a predetermined angle. The determination of tightness by the calculation of the angle provides only a very approximate value because it is difficult to have all the parameters come into play in a uniform manner (the calculation of the rigidity of the joint is delicate).
To determine the predetermined angle, a series of bolts equipped with gauges which measure the prestressed force can be screwed and the average of the angles obtained can be used. Another method consists of using a control joint and a comparator which measures the elongation of the bolt, which is proportional to the prestressed force. The variations of rigidity are small from one sample to another and this method provides better results than the torque control. The main problem being, however, the detection of the rise in torque which serves as a starting point for the evaluation of the angle.
Elastic limit screwing. This method consists of stopping the screwing when the elastic limit of the threaded joint element is reached. To do this, after noting on the curve showing screwing torque as a function of the angle of rotation of the joint element that screwing in the elastic range corresponds to a linear rise in torque, the torque gradient is considered in relation to the angle. The torque gradient is constant in the elastic range and falls suddenly in the plastic range. Therefore, all that is required is to use a relatively simply system to detect the drop of the torque gradient.
However, the use of elastic limit screwing has the drawbacks of requiring that the joint is at its limit, of requiring only the use of screws and bolts whose quality is thoroughly known, and of preventing the insertion of washers because the possible resulting sliding of the joint impairs detection of the gradient drop.